It is preferable to a user that a CD player can be connected to an FM tuner mounted on the user's car and CDs can be reproduced through the FM tuner without using a CD reproducing device.
To meet such a requirement, it has been proposed to provide the audio signal modulating system connected to the FM tuner, where an audio signal from a CD player is modulated so as to be received by the FM tuner for reproducing the CD through the FM tuner.
In the FM broadcasting, the high audio frequencies are emphasized in order to prevent aggravation of the signal-to-noise ratio. Accordingly, an ordinary FM tuner is provided with a de-emphasis circuit for obtaining a flat frequency characteristic. Therefore, if the audio signal from the CD player is fed to the FM tuner without adjusting the level thereof, the high audio frequencies are attenuated by the de-emphasis circuit of the FM tuner, which causes the frequency characteristic to deteriorate.
On the other hand, such an audio signal modulating system must have a frequency modulating circuit for modulating the audio signal. However, if the percentage modulation rate of the modulating circuit exceeds 100 percent, distortion generates in the reproduced sound by the FM tuner.